In order to improve the qualities of a paper or board sheet, the base web of the paper or board sheet is treated in different ways. The goal of each treatment is to improve the strength or printability properties of the produced grade. Strength improvement is principally accomplished by way of surface sizing, wherein the web surface is coated with a strength-improving sizing agent such as a starch solution. Coating is applied, among other reasons, for such purposes as better product brightness, surface impermeability or smoothness, while calendering serves to improve the surface smoothness and gloss.
Conventionally, web treatment is performed after base web formation so that a dry web is treated in separate off-line equipment or, alternatively, in online equipment connected directly after the paper- or boardmaking machine. Herein, the web is dried at least essentially close to its final degree of moisture content, whereby the web must be moistened and redryed particularly during surface sizing and coating, which increases the machine length and energy consumption. As modern paper/boardmaking machines are already equipped with efficient dewatering and drying sections, it would be advantageous to have the web-wetting operations such as surface sizing and coating moved as close as possible to the headbox so as to take place within the wire section or press section, whereby the dewatering and drying of the base sheet having the surface treatment agent applied thereto can be accomplished at least partially simultaneously. Surface sizing and coating performed at the press or wire section would also offer substantial quality benefits inasmuch the penetration of the surface sizing agent into the web takes place in a manner entirely different from that when the treatment agent causing web wetting is applied to an already dried base sheet. The quality of calendering is improved if this step is carried out on a web of higher moisture content, whereby also the outcome of calendering for the most common paper and board grades would benefit from being performed at the press section of a paper/boardmaking machine.
The history of having the base sheet formation and finishing phases chained in two separate steps can be traced to two major factors. Firstly, the formation of the base sheet and its subsequent finishing have traditionally been considered extremely autonomous production phases to be implemented independently from each other. Secondly, paper webs in particular and even board webs are very fragile prior to their drying close to the final solids content, whereby it has not been possible to execute such treatments that cause wetting of the web without jeopardizing web runnability.
From the art is known an embodiment in which surface sizing is carried out using a film-transfer applicator. In a film-transfer application apparatus, a coating film metered very accurately on a rotating film-transfer roll is transferred from the roll to the surface of the running web. Although a film-transfer applicator offers very good runnability and causes a minimal stress on the web, the water permeating the web anyhow weakens its strength. Since the web will not be passed directly from the film-transfer roll onto a supporting element such as a wire, an unsupported gap remains between the film-transfer roll and the subsequent supporting element. Hence, the web is always subjected to stresses in the cross-machine direction and particularly in the machine direction. For instance, variations in the moisture content profile may cause stress peaks on the web that readily break the wet and fragile web.
In addition to the technique of film-transfer application, use of spray application has been proposed in the art, wherein the surface size or coating mix is applied to the web by means of an array of spraying nozzles staggered in the cross-machine and/or the machine direction.
In U.S. Pat. No. 3,146,159 is described an embodiment in which application is performed on a wet web by coating one side of the web and simultaneously supporting the web during application from its other side by a fabric. Coating on a calibrating press is also described.
U.S. Pat. No. 4,793,899 describes spray-coating and short-dwell application techniques, wherein the web support arrangement is more advanced than that of the above-cited patent, however, not even this embodiment is free from unsupported web travel passages and the applicator still has a web-supporting fabric therein.
U.S. Pat. No. 5,152,872, there is described an embodiment free from unsupported web travel passages. In this arrangement, the coating mix is first metered on the outer surfaces of rolls and therefrom directly to the web, yet having a felt running in the nip.